Type III compensation is often used for the voltage-mode control DC/DC switching power converter to achieve wider bandwidth than the inductor-capacitor (LC) resonant frequency of the DC/DC switching power converter. “Demystifying Type II and Type III Compensators Using Op-Amp and OTA for DC/DC Converters”, S. W. Lee, Texas Instruments Application Report—SLVA662, July, 2014, states that the purpose of adding compensation to the error amplifier of a DC/DC switching power converter is to counteract some of the gains and phases contained in the control-to-output transfer function. The gains and phases may jeopardize the stability of the DC/DC switching power converter. The ultimate goal is to make the overall closed-loop-transfer function (control-to-output cascaded with the error amplifier) satisfy the stability criteria.
A Type I compensation has a single pole based on a feedback capacitor and resistor at the input of an operational amplifier or an impedance at an output of an operational transconductance amplifier with the resistor or resistors at the input of the operational transconductance amplifier. A Type II compensation has two poles and adds an resistor-capacitance (RC) branch to flatten the gain, and improve the phase response in the mid-frequency range. The increased phase is achieved by increasing the separation of the pole and zero of the compensation. A Type III compensation has two poles, besides the pole-at-zero and two zeros. The Type III compensation is used when more than 90 degrees of phase boost is necessary. By adding another pole/zero pair to the Type II compensation, the Type III compensation can theoretically boost the phase up to 180 degrees.
FIG. 1a is a block diagram of a switch mode DC/DC power converter of the related art. FIG. 1b is a schematic diagram of a control stage 5 of the switch mode DC/DC power converter of the related art of FIG. 1a. Referring to FIG. 1a, a switch mode DC/DC power converter transfers power from a source VIN to a load while converting voltage and current applied to the input of the circuit to an output voltage VOUT and current IOUT suitable for the load. The switch mode DC/DC power converter consist of a control stage 5 and a power stage 10. The control stage 5 receives necessary feedback signals from the power stage 10 and a reference voltage VREF and control signals from system operating functions. The feedback signal VFB is applied to a compensator 20 to correct the phase and gain of the feedback signal VFB. The compensated feedback signal VCTRL is applied to an error amplifier 15 for determining the difference between the compensated feedback signal VCTRL and the reference voltage VREF. The output terminal 7 of the error amplifier 15 transfers the difference output voltage VDIF to be applied to the modulator 25 of the power stage 10. The modulator 25 compares the difference output voltage VDIF with a ramp voltage VRAMP to determine a pulse width of the modulated input voltage VMOD. The modulated input voltage VMOD is applied to the filter 35 for removing the high frequency content from the modulated input voltage VMOD for determining the output voltage VOUT of the switch mode DC/DC power converter.
Referring to FIG. 1b, the error amplifier 15 of the control stage 5 has a transconductance amplifier 17. The transconductance amplifier 17 receives the feedback signal VFB at its inverting terminal (−) and the reference voltage VREF at its noninverting terminal (+). The output of the transconductance amplifier 17 is connected to a first terminal of the feedforward resistor Rff and the first terminal of the compensation capacitor Cc. The second terminal of the compensation capacitor Cc is connected to the ground reference voltage. The second terminal of feed forward resistor Rff is connected to the output terminal 7 of the error amplifier 15 for providing the difference output voltage VDIF to the power stage 10.
The compensator 20 adds feedforward compensation that increase the phase margin, defined as the difference between the unity-gain phase shift and −180 degrees, which is the point where the loop becomes unstable.
The finite gain or low gain amplifier 22 receives the feedback signal VFB at its inverting terminal (−) and the reference voltage VREF at its noninverting terminal (+). The output of the finite gain amplifier 22 is connected to the first terminal of the feedforward capacitor Cff. The second terminal of the feedforward capacitor Cff is connected to the second terminal of the feed forward resistor Rff and connected to the output terminal 7 of the error amplifier 15 for providing the difference output voltage VDIF to the power stage 10.
FIG. 1c is a plot of gain and phase vs. frequency of the control stage 5 of the switch mode DC/DC power converter of the related art of FIG. 1a. The plot 40 is the gain of the error amplifier 15 and the plot 45 is the gain of the compensator 20. The total gain of the control stage 5 is shown in the plot 50. The compensator 20 forms a Type III compensation that is often used for the switch mode DC/DC power converter to achieve wider bandwidth than the load inductor-capacitor (LC) resonant frequency. The two zeros 55 cancel the resonant frequency of the inductor and output capacitor, and 0-dB frequency of the whole control stage 5 can be higher than the resonant frequency. However, the feed-forward path of the compensator 20 is effective only during the feed-forward pole time constant (about 3 Cff Rff) and also the dynamic range is limited by the power supply voltage source VDD.
FIG. 2 is a plot of the large signal response of the error amplifier 15 and output voltage VOUT of the switch mode DC/DC power converter of the related art of FIG. 1a. The plot 65 shows the output voltage VOC response of the compensator 20 to a large load and/or line transient. The plot 70 shows the output voltage VOEA of the error amplifier 15 and the plot 75 illustrates the output voltage VDIF of the control stage 5. The output voltage VDIF of the control stage 5 is the difference voltage between the reference voltage VREF and the voltage level of the compensated feedback signal VCTRL.
When a load and/or line transient is large and/or long and the output voltage VOUT cannot be regulated while the feed-forward path of the compensator 20 is effective, the transient speed is restricted by the main pole, which is very slow, and an overshoot or undershoot 80 of the output voltage VOUT during the transient becomes large in the time prior to the time τ0. When the output voltage VOC response of the compensator 20 no longer increases, the output voltage VOEA of the error amplifier 15 begins to dominate at the time τ0 and the output voltage VOUT of the switch mode DC/DC power converter begins to raise to its required voltage level and the switch mode DC/DC power converter becomes regulated at the time τ1.